A seat belt for restraining a vehicle occupant from being thrown from the seat of the vehicle in case of a vehicle collision is required to be capable of positively restraining the occupant and yet permitting some freedom of motion to the occupant. A spring loaded retractor having an inertia lock mechanism serves such a purpose but a slight pay out of the belt from the retractor is unavoidable in case of a collision and some slack is always present even before the occurrence of a collision. Such a slack in the seat belt is undesirable because upon occurrence of a collision the occupant is initially thrown forward as far as permitted by the slack of the seat belt and then abruptly stopped by the seat belt. Therefore, if an excessive slack is present in the seat belt, the seat belt is incapable of achieving its full capability to positively restrain the vehicle occupant in case of a vehicle collision. Therefore, it is desirable to reduce any such slack in the seat belt to a smallest possible level so as to reduce such a delayed action of the seat belt to restrain the vehicle occupant.
Japanese patent laid open publication No. 60-259553 proposes a device for tightening a seat belt upon detection of a certain level of deceleration. U.S. Pat. No. 4,580,810 discloses a passive occupant restraint system in the form of an air bag fitted into a steering wheel so as to rapidly inflate upon detection of a collision.
In any of these passive occupant restraint systems, it is essential for the system to be able to be activated without fail when required and otherwise stay dormant because unnecessary activation of such a system not only imposes a great inconvenience on the occupant but also may prevent the driver from handling the vehicle in a safe fashion.
In copending patent application No. 132,257 filed Dec. 14, 1987, there is proposed a deceleration sensor which comprises a pair of sensor masses which act upon a single trigger member with the aim of improving the reliability of the action of the sensor when a level of deceleration indicative of a vehicle collision is detected. The contents of this copending application are incorporated herein by reference.